1. Field of the Invention
The present invention relates to a receiving-layer-forming resin for a thermal-transfer image-receiving sheet, which is superposed on a thermal-transfer sheet and used, and also concerns a thermal-transfer image-receiving sheet using such a resin.
2. Description of the Related Art
With respect to an image-forming method in which thermal transferring processes are utilized, a method (sublimation-type thermal transfer system) in which a thermal transfer sheet formed by allowing a substrate sheet such as paper and a plastic film to support a sublimable dye serving as a recording material and a thermal transfer image-receiving sheet in which a receiving layer for the dye is formed on paper or a plastic film are superposed on each other so that a full-color image is formed has been known. In this method, since the sublimable dye is used as the color material, the density and tone are freely adjustable on a dot basis so that it is possible to clearly form a full-color image faithful to an original document on an image-receiving sheet; therefore, this method is applied to color-image forming processes in apparatuses such as digital cameras, videos and computers. The resulting image is a high-quality image that corresponds to a silver-salt photograph.
In recent years, there have been increasing demands for high-quality images, and images, which have not only a high dyeing property (sensitivity) but also high environmental resistance typically represented by light resistance to light exposure and anti-fingerprint property, have been required. With respect to a receiving-layer-forming resin for the thermal-transfer image-receiving sheet, resins mainly containing a saturated polyester resin have been proposed (for example, see Japanese Patent Application Laid-Open Nos. 57-107885, 60-64899 and 62-105689). However, one of problems with the conventional polyester resins is that it is difficult to sufficiently satisfy both of the above-mentioned properties.
With respect to a binder for the thermal transfer sheet, an acetal-based resin (in particular, acetal resin) has been mainly used, and in the case when a polyester-based resin is used as the receiving layer, the acetal-based resin and a polyester-based resin tend to be thermally fused and joined to each other due to heat generated by a thermal head upon printing, and the resulting problems are that a big peeling sound is generated upon peeling, that dye layer itself is transferred, and that a so-called abnormal transfer in which the receiving layer is peeled from the substrate tends to occur; consequently, it is difficult to maintain a desirable releasing property between the thermal-transfer sheet and the image-receiving sheet.
In order to ensure a sufficient releasing property, for example, techniques in which a large amount of a releasing agent such as silicone is added to the receiving layer have been proposed (for example, Japanese Patent Application Laid-Open Nos. 08-108636 and 2002-264543). However, these techniques cause problems such as a poor printed-article preserving property and a failure in transferring the protective layer although a sufficient releasing property is obtained, making it difficult to maintain the releasing property and the protective-layer transferring property in a well-balanced manner.